Google’s Click to Call program has been a huge success. About a year ago Google reported that it had 500,000 advertisers using Click to Call. (That number was repeated again today in a blog post.) And last year former Google Product SVP Jonathan Rosenberg said “Click-to-Call ads are generating millions of calls every month.”

Calls and call metrics are increasingly important to Google because they’re a form of “offline” conversion tracking that provides more visibility on the true efficacy of keywords and campaigns. Selling calls also enables Google to drive phone-based leads, which are more qualified and often of higher value than clicks.

Google’s Click to Call program has relied to date on AdWords phone extensions. Phone numbers in mobile ads (on smartphones) are highlighted, users click them and initiate calls. Those calls are tracked. Both small merchants and national advertisers have used the ads. But national advertisers, such as Direct TV and security company ADT, have been among the most enthusiastic adopters of Click to Call because they can deliver “warm leads” to call centers, which can more efficiently close sales.

Today Google announced that is broadening mobile call tracking with a new capability that reaches beyond phone numbers in AdWords to mobile landing pages. Google is “introducing a new conversion tracking metric . . . to report calls placed from mobile pages.” In other words Google will be tracking calls made after the click-through to a site.

For the present the new tracking capability is free. On the PC Google charges $1 per completed call for its call tracking service. I’m speculating by analogy that Google may eventually charge a fee for the service. They have not indicated to me that they will however.

Marketers can use existing phone numbers or a “call button” on a mobile site or landing page. They don’t need a specialized call tracking number from Google:

You can start using this new metric by installing a snippet of code on your landing pages. Upon doing so, you can set ‘calls’ as a conversion metric to track on your landing page. Next, you can pair calls focused campaigns with tools like Conversion Optimizer, then bid by setting a target CPA for calls, and ultimately automate ad serving to optimize for calls.

Call tracking and metrics provider Telmetrics recently introduced a very similar capability:Telmetrics Introduces Call Tracking Without Tracking Numbers. And Marchex, which also provides call-based advertising, recently released findings of an advertiser and agency surveyshowing nearly 50% of respondents wanted to generate calls from their digital advertising (PC and mobile).

The Marchex survey data also showed marketers assign greater value to campaigns and channels when they deliver calls vs. other types of leads or actions (e.g., clicks, form fills).